The present invention relates to a disk drive apparatus for computer or other data processing applications, whereby automatic correction of off-track errors of a read/write head with respect to recording tracks formed on a surface of the disk is implemented, without the necessity to provide dedicated servo tracks or embedded servo data regions on the disk.
In recent years there has been a trend towards increasing the density of tracks which are provided on recording disks of the type generally referred to as "floppy disks". These are formed as flexible sheets and are used as an external recording medium for computers etc. To attain higher track densities, there is a requirement for an increase in the accuracy to which the read/write head (generally an electromagnetic head) used with such a disk will follow the recording tracks upon the disk, i.e. a requirement for elimination of off-track errors (that is, errors between the actual radial position of a read/write head and the optimum position of that head with respect to a track). Various tracking servo systems have been proposed in the prior art, for increasing this tracking accuracy. With one such system, outer and inner dedicated servo tracks are respectively provided around the outer periphery and inner periphery of the data tracks which are recorded on the disk, while in addition an optical scale is incorporated. The optical scale is used to accurately detect the position of the read/write head with respect to the reference tracks, whereby compensation is executed for any off-track error of the read/write head position.
With another known method employing inner and outer dedicated servo tracks, a stepper motor having a fine step adjustment capability is employed to move the read/write head. When a disk is newly set in the apparatus, the head is first moved into position over the outer servo track, and fine step adjustment is then performed to position the head directly over that track, by a servo positioning loop which functions on the basis of the amplitude of a signal which is read out from the head. The head is then moved to the inner servo track, and servo operation is again performed, to position the head directly over that track. The amount of fine adjustment of the head position which is required to align the head directly above the inner and outer servo tracks is thereafter utilized as data for determining amounts of fine adjustment which must be performed for each of the data tracks, to attain accurate head positioning for each track.
With another tracking servo method which is known in the prior art, a train of pulse signals are recorded as servo data in a predetermined region in each of the data tracks, i.e. as embedded servo data. The read signal level which results from reproduction of these pulse signals is compared with a reference level, whereby the amount of off-track error and the direction of that error can be detected. Compensation for this off-track error is performed by tracking servo operation.
With the prior art tracking servo arrangements describe above, it is necessary that dedicated servo tracks or embedded servo data regions be recorded beforehand upon each of the disks. This leads to increased manufacturing cost, and in addition such a method renders it impossible to utilize disks which do not have such dedicated servo tracks or embedded servo data regions recorded thereon. Furthermore, if such special disks are employed, then if the dedicated servo tracks or embedded servo data regions are inadvertently erased, tracking servo operation will no longer be possible.
Also, in the case of the second prior art method described above, continuous servo operation is not performed after the initial correction data has been obtained, based on the inner and outer servo tracks. Thus, any variation in recording track position which cannot be determined from that initial correction data (e.g. a shift in track position due to thermal expansion, for example) cannot be corrected with such a system.
The various sources and amounts of off-track error are graphically illustrated in FIG. 2. As shown, these include radial expansion and contraction of the disk due to changes in temperature or humidity, deviations in dimensions of different read/write heads (i.e. between different disk drive units), etc. However as can be understood from FIG. 2, the major source of off-track error is radial expansion and contraction of the disk, which can result in large amounts of off-track error along a direction extending from the inner periphery to the outer periphery of the disk recording area.
It can thus be understood that it is essential, for a disk drive apparatus providing a very high recording track density, to continuously monitor the position of the read/write head with respect to a track which is currently being accessed.